1. Field of the Invention
This invention relates to a structure of a rotor for a generator comprising a rotor formed of a permanent magnet mounted on a rotary shaft, and a stator disposed around an outer circumference of the rotor; and a method of manufacturing the rotor.
2. Description of the Prior Art
As the performance of a permanent magnet has been improved, the opportunities in which a permanent magnet is used as a rotor of a generator have increased. A generator-motor using a permanent magnet as a rotor obtains high generating and power efficiencies, and can be formed to a simple structure. Owing to these characteristics, such a generator-motor has recently come to be used much in industrial machines and tools.
When a rotational frequency of a rotor in a conventional generator-motor increases as a voltage and an amperage increase, a large centrifugal force occurs in the rotor, so that the rotor is required to withstand the centrifugal force. Therefore, an outer circumference of the permanent magnet constituting the rotor is reinforced with a reinforcement ring so that the rotor can withstand the centrifugal force.
The known generators using a permanent magnet as a rotor include, for example, the miniaturized generator disclosed in Japanese Utility Model Laid-Open No. 146975/1990, the high output AC generator disclosed in Japanese Patent Laid-Open No. 236260/1995, the permanent magnet type rotary machine disclosed in Japanese Patent Laid-Open No. 272850/1987 or the dynamo-electric machine disclosed in Japanese Utility Model Laid-Open No. 162977/1985.
In the miniaturized generator disclosed in Japanese Utility Model Laid-Open No. 146975/1990, a main shaft and a rotor are connected together via a governor mechanism in which a governor weight is supported pivotably on a pair of links so that the rotor is moved in the direction in which the rotor comes out of a stator by separating the governor weight from the main shaft by a centrifugal force in accordance with the rotational frequency of the main shaft and thereby reducing an angle between the links.
The high output AC generator disclosed in Japanese Patent Laid-Open No. 236260/1995 is adapted to control a generating power properly by controlling a magnetic flux density in accordance with a rotation speed thereof, and provided with a control ring relatively rotatably between a rotor and a stator, and a permeable member engageable with and disengageable from the control ring.
In the permanent magnet type rotary machine disclosed in Japanese Patent Laid-Open No. 272850/1987, a permanent magnet is provided on a rotor, and also a pole piece-forming container adapted to guide a movable magnetic member diametrically by the rotation of the rotor in which the movable magnetic member is sealed.
The dynamo-electric machine disclosed in Japanese Utility Model Laid-Open No. 162977/1985 is provided on its rotatable support shaft with a magnetic flux density variable mechanism capable of varying the magnetic flux density of a magnetic circuit formed by a permanent magnet corresponding to the rotational frequency of the support shaft.
As mentioned above, in a generator formed by using a permanent magnet as a rotor rotating at a high speed, a permanent magnet constituting a rotor is produced mostly by sintering powder of an iron-neodymium alloy, and forming a sintered body thus obtained to a predetermined shape and precision. However, it is difficult to grind a permanent magnet comprising such a sintered body by a grinding tool due to the material of the permanent magnet. This causes the processing time for forming a permanent magnet into a rotor, a manday for subjecting a permanent magnet to various steps from a grinding step to a precision processing step and the rotor manufacturing cost to increase. A permanent magnet comprises a sintered body of an alloy of rare earth metals, and has a low processability. Since this sintered body is ground slowly so as to prevent a bit from being damaged, the processing cost increases greatly as compared with the material cost, so that the manufacturing cost becomes high.
In a generator using a permanent magnet as a rotor, generated power is represented by a product of a rotation speed of the rotor and the intensity of a magnetic field, and, therefore, the generated power increases in proportion to the rotation speed of the rotor. The intensity of the magnetic field is represented by a product of a magnetic force of the permanent magnet and an area thereof. When the rotational frequency of the rotor increases in accordance with an increase in the voltage and amperage, a large centrifugal force occurs therein. Since the rotor is broken if it does not withstand such a centrifugal force, so that the rotor is required to resist the centrifugal force. To meet the requirement, a permanent magnet constituting such a rotor is generally reinforced at an outer circumference thereof with a reinforcement ring so that the rotor can withstand the centrifugal force.
A cylindrical permanent magnet is usually made by filling a cylindrical mold of a nonmagnetic material with alloy powder containing elements, such as iron, neodymium, samarium and cobalt, solidifying the alloy powder by compression molding the same at a high temperature, and instantaneously sintering the resultant molded body by high-frequency heating with the NS poles, i.e., lines of magnetic force in the alloy aligned by applying a magnetic force thereto during the sintering of the molded body. After a magnetic force is applied to the molded body, a cylindrical sintered body of a permanent magnet is taken out of the mold, and the outer and inner circumferential surfaces thereof are ground. On the other hand, a thin-walled outer cylinder comprising wound carbon continuous fiber is formed as a member for reinforcing the sintered body. The sintered body of a permanent magnet is press fitted in the thin-walled outer cylinder to finish a rotor.
However, it is difficult and takes a long time to grind the outer and inner circumferential surfaces of a sintered body of a permanent magnet, and this causes the manufacturing cost to increase greatly. It is difficult to form a permanent magnet cylindrically to a large diameter, and a permanent magnet into a large-diameter rotor. Consequently, large output generation cannot be expected as long as a permanent magnet is used as a rotor. Therefore, issues to be resolved are how to develop a method of grinding a sintered body of a permanent magnet easily, speedily and precisely so as to reduce the manufacturing cost.
Since the cost of the material for a permanent magnet, such as iron, neodymium, samarium and cobalt is high, utilizing the material efficiently is desired. As the dimensions of a generator increase, those of a permanent magnet have to be increased. To meet this requirement, using segmented permanent magnet can be conceived. When a permanent magnet is divided into segment members, in to what type of a structure should these members be combined is an issue.
When a rotor in a generator is rotated at a high speed, a large centrifugal force is exerted thereon. Therefore, it is always demanded that a permanent magnet be reinforced for the retention of the same, i.e., for preventing the permanent magnet from being bursted by the centrifugal force. Consequently, studying a structure capable of increasing the strength of a permanent magnet is required. When a rotor in a generator is rotated at a high speed, a high output is obtained, and this poses an issue as to what structure is required for a rotor having smaller weight, a higher rigidity and a higher resistance to a high-speed rotation.
When a large output is obtained from a generator by rotating a rotor, which comprises a permanent magnet, at a high speed, the temperature increases due to an iron loss and a copper loss, and the permanent magnet is demagnetized due to the heat generated. When demagnetization occurs in the permanent magnet constituting a rotor, the permanent magnet is decomposed, so that it has to be remagnetized. Consequently, when the dimensions of the rotor increases, the cost of regenerating the permanent magnet increases correspondingly, and a loss becomes larger. Therefore, when a rotor is formed of a permanent magnet, it is necessary that the rotor be cooled so that demagnetization does not occur in the permanent magnet, and developing the techniques for cooling a rotor effectively is an issue encountered in the prior art.